Analysis and comparison of control strategies for normal adjustment of a robotic drilling end-effector

Robotic drilling technology for aircraft flexible assembly has challenges and is under active investigation. In this work, a robotic drilling end-effector is designed and its normal adjustment system is dynamically modeled for comparison of advanced control strategies in terms of position tracking precision and dynamic quality. Three control algorithms with different computational complexity are proposed and compared: Based on computation torque control method first, a proportional and differential controller (PDC) and a sliding mode controller (SMC) are proposed respectively, and then is a model reference adaptive controller (MRAC). Simulation results show that the SMC has higher precision and a more excellent tracking property than the PDC of which the proportional and derivative gains have been optimally tuned using a modified Ziegler-Nichols’ (Z-N) tuning methods. An experiment platform is established in MatLab xPC environment to validate the effect of the SMC and MRAC. The experiment results show that the MRAC delivers a better robust performance that allows adaptiveness to the nonlinear factors such as disturbance and parameter variations than the SMC

Effect of joint nonlinearities on the dynamic performance of machine tools.

Evaluating the dynamic performance of machine tools at the design stage requires prediction of Frequency Response Functions (FRFs) and Stability Lobe Diagrams (SLDs). This is, however, a challenge because of nonlinearities in the machine joints. Here we address this challenge by providing a method to identify such joint nonlinearities and use them to predict the FRFs and SLDs for a machine tool design concept. An experimental study was undertaken to understand the relationship between the machine structure and its dynamic characteristics during cutting using the Arch-Type Reconfigurable Machine Tool. It has been experimentally determined that the machine performance is insensitive to the structural changes if the dominant frequency in the machine's FRF arises primarily due to the tool-tool holder-spindle assembly. The FRFs and the analytical SLDs for the Arch-Type Reconfigurable Machine Tool were evaluated and it was found that the machine had similar characteristics at various reconfiguration positions. Linear techniques to predict FRFs at the design stage can be inaccurate as even weak nonlinearities in machine tool joints can lead to significant changes. Identification of machine joint nonlinearities is difficult as they are typically very stiff and, thus, any dynamic experiment to identify joint nonlinearities has to be done at very high frequencies where the results are sensitive to small errors in calibration and geometry. Guidelines to carry out such experiments have been discussed, and the experiments were carried out on a translational guide of the type often used in machine tools. Both a parametric and nonparametric approach has been used to determine the nonlinear restoring force relationship for the translational guide. This relationship is then converted into the frequency domain using describing functions. A derivation is presented to convert the two dimensional polynomial result for determined nonlinear restoring force relationship from nonparametric approach into a closed-form describing function expression. Finally the effects of the experimentally determined nonlinearities in the translational guide on overall machine structural dynamics is determined using the nonlinear receptance coupling approach. Significant variations are observed in both FRFs, as well as the SLDs for a column-spindle head machine structure even with weakly nonlinear joints. Such variations in natural frequency and response of the FRF, and in the location of stability lobes with respect to the spindle speed, were also experimentally observed for the Arch-Type Reconfigurable Machine Tool.Ph.D.Applied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126409/2/3253259.pd

A time-domain fault detection method based on an electrical machine stator current measurement for planetary gear-sets

Fault diagnosis of geared drive-train systems is usually based on vibration monitoring. However, such vibration based techniques are difficult to implement in planetary gearboxes due to the complex nature of measured vibration spectrum. Motor current signal analysis (MCSA) provides an alternative and non-intrusive way to detect mechanical faults through electrical signatures. In this paper, a new time-domain fault detection algorithm is presented for the detection of planetary gear faults using electrical machine stator current signals. This time-domain fault detection method combines fast dynamic time warping (DTW) and correlated kurtosis techniques to process the current signals data to detect and identify damaged planetary gear and its position. Fast DTW is employed to highlight the sideband patterns resulting from tooth damage by the introduction of an estimated reference signal that has the same frequency as the gear mesh frequency. Correlated kurtosis (CK) takes advantages of the periodicity of the geared faults; it is used to identify the position of the damaged gear tooth in the planetary gear-set. This method is later applied to simulated current signals generated from a lumped parameter model of planetary gearbox driving a permanent magnet synchronous generator to evaluate its performance. The simulated results demonstrate the effectiveness of the proposed time-domain approach to detect faults in planetary gear-sets based on the electrical stator current signal

Correlation-based estimation of cutting force coefficients for ball-end milling

This article presents a methodology to estimate cutting force coefficients based on the least squares approximation using correlation factor between the estimated and measured cutting forces in order to determine the corresponding tool angular position. This method can be applied on measured cutting force data over any small interval of time that need not contain information of the time instant when the cutting tool enters the workpiece, which has been the main requirement in the conventional method. This allows a quick estimation of the cutting force coefficients regardless of the chosen cutting conditions and tool-workpiece material, which is often the case in industrial machining processes. This proposed method has been validated by comparison of cutting force coefficients obtained using conventional estimation technique for a slot ball-end milling test. Besides being useful for predictive evaluation of forces, such estimation of cutting force coefficients of the cutting force model can be useful for understanding variations in cutting process over the tool life and can assist in online monitoring and process optimization

Collective pitch control of wind turbines using stochastic disturbance accommodating control

Fidelity of a plant's dynamic model is a concern in any controller design process. In this context, fidelity refers to which dynamics of the plant needs to be included in the control model and which dynamics can be left out or approximated. Studies on wind turbine control have shown that modelling error due to the unmodeled dynamics can lead to unstable closed-loop dynamics. This paper investigates the use of Kalman estimator to design the Stochastic Disturbance Accommodating Control (SDAC) scheme to stabilize the system in the presence of the unmodeled dynamics. Performance of the presented control scheme is investigated through simulations on two different wind turbine configurations under turbulent wind conditions with different mean wind speeds and turbulence intensities using FAST (Fatigue, Aerodynamics, Structures, and Turbulence) aero-elastic tool. The generator speed regulation, drivetrain load, and control effort of the presented control scheme are compared with those of the baseline Gain Scheduled Proportional Integral (GSPI) controller. The results indicate better speed regulation and lower drivetrain load for the presented SDAC under the tested wind conditions

Effect of side edge angle and effective rake angle on top burrs in micro-milling

Experimental investigations on the effect of side wall edge strengthening, to reduce top burr formation, in micro-milling slots in Al-6061 alloy using a carbide tool are reported here. The side edge is strengthened by increasing the edge angle to a value higher than the usual 90°. Side edge angle is varied in two ways: one, by changing the work geometry and two, by introducing a taper into the milling tool. The burrs formed are examined qualitatively in a scanning electron microscope and quantitatively using a surface profiler. The analysis of the results shows that top burrs are reduced both by strengthening the side edge and also by the effect of the taper angle in the micro-milling tool. The effect of the side edge angle in the tool can be attributed to the edge strengthening. On the other hand, an analysis of the tapered tool geometry indicates that the velocity rake, normal rake and effective rake angles increase with the taper angle and can hence explain the observed burr reductions

An Active Control with a Magnetorheological Damper for Ambient Vibration

The ambient vibration in manufacturing and assembly plants caused by nearby large equipment or heavy vehicles can produce dynamic machining error and even generate chatter in machining systems such as robotic drilling systems. In this paper, we present an active control method with a magnetorheological damper (MRD) for reducing ambient vibration in a robotic machining system, with the advantages of wider frequency bandwidth and robustness. A sliding mode control (SMC) algorithm is proposed as well. The control performance of the SMC under different excitations is simulated by Simulink and compared with that of the PID control algorithm; the result shows that the SMC is superior to the PID control and passive vibration control. An MRD is designed based on the control force of the active vibration control in the time domain in order to provide the required damping force. The results of co-simulation using ADAMS and Simulink verify that the ability of the SMC to control vibration performance is significantly improved compared with that of the passive vibration control

An Active Control with a Magnetorheological Damper for Ambient Vibration

The ambient vibration in manufacturing and assembly plants caused by nearby large equipment or heavy vehicles can produce dynamic machining error and even generate chatter in machining systems such as robotic drilling systems. In this paper, we present an active control method with a magnetorheological damper (MRD) for reducing ambient vibration in a robotic machining system, with the advantages of wider frequency bandwidth and robustness. A sliding mode control (SMC) algorithm is proposed as well. The control performance of the SMC under different excitations is simulated by Simulink and compared with that of the PID control algorithm; the result shows that the SMC is superior to the PID control and passive vibration control. An MRD is designed based on the control force of the active vibration control in the time domain in order to provide the required damping force. The results of co-simulation using ADAMS and Simulink verify that the ability of the SMC to control vibration performance is significantly improved compared with that of the passive vibration control

Defect Characteristics and Online Detection Techniques During Manufacturing of FRPs Using Automated Fiber Placement: A Review

Automated fiber placement (AFP) is an advanced manufacturing method for composites, which is especially suitable for large-scale composite components. However, some manufacturing defects inevitably appear in the AFP process, which can affect the mechanical properties of composites. This work aims to investigate the recent works on manufacturing defects and their online detection techniques during the AFP process. The main content focuses on the position defect in conventional and variable stiffness laminates, the relationship between the defects and the mechanical properties, defect control methods, the modeling method for a void defect, and online detection techniques. Following that, the contributions and limitations of the current studies are discussed. Finally, the prospects of future research concerning theoretical and practical engineering applications are pointed out